1. Field of the Invention
This invention is concerned with methods of fabricating microelectronic structures using EUV (extreme ultraviolet) lithography.
2. Description of the Prior Art
As the semiconductor industry continues to follow Moore's law, the demand for ever-decreasing feature sizes requires the use of thinner films to prevent pattern collapse. Thinner films will require using a hardmask to transfer the pattern to the substrate. Extreme ultraviolet (EUV) exposure is expected to be the method of choice for single exposure lithography to achieve the required critical dimension (CD) targets of the 22-nm node and beyond. Unfortunately, EUV lithography has been hindered by a number of problems, one of the most notable being a lack of powerful radiation sources. Because of the limited amount of energy available, lithography performance can suffer, and long exposure times are needed, resulting in low throughput. One solution to this problem is to improve the sensitivity of EUV resists. This sensitivity can be further defined by the amount of energy, also called dose, needed to reach a certain feature size, or more simply put, dose to size.
The main challenge in material design, particularly with resists, is to simultaneously achieve the improvement in line width roughness (LWR) and sensitivity necessary for industry targets. LWR is a measure of the variation of the width of the lines formed by photolithography. Sensitivity is the minimum dose of energy necessary to image the photoresist. The International Technology Roadmap for Semiconductors has set targets for 3 sigma LWR at <3 nm, and for sensitivity at <10 mJ/cm2. Most currently available resists do not meet both targets and tend to achieve a compromise between LWR and sensitivity. Numerous attempts have been made to improve resist sensitivity while improving LWR. Among the various approaches is incorporating a photoacid generator (PAG) functionality into the resist polymer. Such methods are limited to the maximum or minimum amount of PAG that must be present in the polymer formulation for optimum lithographic performance. It is also synthetically challenging to obtain the ideal PAG composition for incorporation into a polymer backbone.
In another approach for resist design, halogens have been used in an attempt to increase resist absorbance to reduce LWR and increase sensitivity. This approach is promising, but the resist needs a certain level of transparency to minimize tapering that can occur at the bottom of the resist. Tapering is caused by the difference in light absorption that occurs as photons travel from the top of the resist to the bottom of the resist. It has also been suggested that resist absorbance and secondary electron generation by EUV irradiation play an important role in acid generation that can improve LWR and sensitivity. However, as discussed above, there are limitations on the increase in absorbance that can be achieved in a resist film before tapering occurs.
There is a need for EUV methods that improve LWR, while simultaneously having the necessary sensitivity.